An irregular galaxy is a galaxy that does not have a distinct regular shape, unlike a spiral or an elliptical galaxy. The shape of an irregular galaxy is uncommon – they do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure. Collectively they are thought to make up about a quarter of all galaxies. Most irregular galaxies were once spiral or elliptical galaxies but were deformed by disorders in gravitational pull. Irregular galaxies may contain abundant amounts of gas and dust. This is not necessarily true for Dwarf Irregulars. 
There are two major Hubble types of irregular galaxies:
- An Irr-I galaxy (Irr I) is an irregular galaxy that features some structure but not enough to place it cleanly into the Hubble sequence. De Vaucouleurs subtypes this into galaxies that have some spiral structure Sm, and those that do not Im.
- An Irr-II galaxy (Irr II) is an irregular galaxy that does not appear to feature any structure that can place it into the Hubble sequence.
A third classification of irregular galaxies are the dwarf irregulars, labelled as dI or dIrrs. This type of galaxy is now thought to be important to understand the overall evolution of galaxies, as they tend to have a low level of metallicity and relatively high levels of gas, and are thought to be similar to the earliest galaxies that populated the Universe. They may represent a local (and therefore more recent) version of the faint blue galaxies known to exist in deep field galaxy surveys.
Some of the irregular galaxies are small spiral galaxies that are being distorted by the gravity of a larger neighbor.
The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm  a type of barred spiral galaxy, the barred Magellanic spiral type. The Small Magellanic Cloud remains classified as an Irregular galaxy of type Im under current Galaxy morphological classification, although it does contain a bar structure. Therefore, newer classification schemes place the SMC outside the irregular class as well.
See also 
- Butz, Stephen D. (2002). Science of Earth Systems. Cengage Learning. p. 107. ISBN 978-0-7668-3391-3.
- Morgan, W. W. & Mayall, N. U. (1957). "A Spectral Classification of Galaxies." Publications of the Astronomical Society of the Pacific. 69 (409): 291–303.
- Faulkes Telescope Educational Guide - Galaxies - Irregulars
- Walter, F. et al Astophys J 661, 102 - 114, 2007
- Gallagher, J. S. & Hunter, D. A. (1984). "Structure and Evolution of Irregular Galaxies." Annual Review of Astronomy and Astrophysics. 22: 37-74. doi:10.1146/annurev.aa.22.090184.000345
- Grebel, Eva K. (2004). The evolutionary history of Local Group irregular galaxies. in McWilliam, Andrew; Rauch, Michael (eds) Origin and evolution of the elements. Cambridge University Press. p. 234-254. ISBN 978-0-521-75578-8.
- Corso, G. and Buscombe, W. The Observatory, 90, 229 - 233 (1970) On the spiral structure of the Large Magellanic Cloud